Serotonin has broad activities in the brain, and genetic variation in serotonin receptors and the serotonin transporter, which facilitates reuptake of serotonin into presynapses, have been implicated in neurological diseases. Drugs targeting serotonin-induced pathways are being used in the treatment of many psychiatric disorders, and one focus of clinical research is the influence of genetics on serotonin action and metabolism in psychiatric settings. Such studies have revealed that the variation in the promoter region of the serotonin transporter protein accounts for nearly 10% of total variance in anxiety-related personality, and the effect of this gene on depression was found to interact with the environment.

Levels of serotonin in the brain show association with aggression (Caspi et al. 2002), and a mutation in the gene which codes for the 5-HT2A receptor may double the risk of suicide for those with that genotype.

Using the ultimatum game as model, it was shown that people whose serotonin levels have been artificially lowered will reject unfair offers more often than players with normal serotonin levels.

In addition, serotonin is also a peripheral signal mediator. It is found extensively in the human gastrointestinal tract as about 80-90% of the body's total serotonin is found in the enterochromaffin cells in the gut. In the blood, the major storage site is platelets, which collect serotonin for use in mediating post-injury vasoconstriction.

Recent research suggests that serotonin plays an important role in liver regeneration and acts as a mitogen (induces cell division) throughout the body.

Serotonin and SIDS

Defective signalling of serotonin in the brain may be the root cause of sudden infant death syndrome (SIDS), Italian researchers have found. Scientists from the European Molecular Biology Laboratory in Monterotondo, Italy, genetically modified lab mice to produce low levels of the brain signaling protein serotonin. The results showed the mice suffered drops in heart rate and other symptoms of SIDS, and many of the animals died at an early age.

Researchers now believe that low levels of serotonin in the animals' brainstems, which control heartbeat and breathing, may have caused sudden death, researchers said in the July 4, 2008 issue of Science.

Anatomy

Gross anatomy

The neurons of the raphe nuclei are the principal source of 5-HT release in the brain.
The raphe nuclei are neurons grouped into about nine pairs and distributed along the entire length of the brainstem, centered around the reticular formation.

Thus, activation of this serotonin system has effects on large areas of the brain.

Microanatomy

Serotonin is released from serotonergic varicosities (swellings) into the extra neuronal space, but not from synaptic terminal boutons as other neurotransmitters. Serotonin diffuses over a relatively wide gap (>20µm) to activate 5-HT receptors located on the dendrites, cell bodies and presynaptic terminals of adjacent neurons.

Serotonin taken orally does not pass into the serotonergic pathways of the central nervous system because it does not cross the blood-brain barrier. However, tryptophan and its metabolite5-hydroxytryptophan (5-HTP), from which serotonin is synthesized, can and do cross the blood-brain barrier. These agents are available as dietary supplements and may be effective serotonergic agents.

One product of serotonin breakdown is 5-Hydroxyindoleacetic acid (5 HIAA), which is excreted in the urine. Serotonin and 5 HIAA are sometimes produced in excess amounts by certain tumors or cancers, and levels of these substances may be measured in the urine to test for these tumors.

Antidepressants

The MAOIs prevent the breakdown of monoamine neurotransmitters (including serotonin), and therefore increase concentrations of the neurotransmitter in the brain. MAOI therapy is associated with many adverse drug reactions, and patients are at risk of hypertensive emergency triggered by foods with high tyramine content and certain drugs.

Some drugs inhibit the re-uptake of serotonin, making it stay in the synapse longer. The tricyclic antidepressants (TCAs) inhibit the re-uptake of both serotonin and norepinephrine. The newer selective serotonin re-uptake inhibitors (SSRIs) have fewer side-effects and fewer interactions with other drugs.

Antiemetics

5-HT3 antagonists such as ondansetron, granisetron, and tropisetron are important antiemetic agents. They are particularly important in treating the nausea and vomiting that occur during anticancer chemotherapy using cytotoxic drugs. Another application is in treatment of post-operative nausea and vomiting. Applications to the treatment of depression and other mental and psychological conditions have also been investigated with some positive results.

Recent research conducted at Rockefeller University shows that both in patients who suffer from depression and in mice that model the disorder, levels of the p11 protein are decreased. This protein is related to serotonin transmission within the brain.

Obsessive-compulsive disorder (OCD) can be a debilitating disorder with the following two anxiety-related essential features: obsessions (undesirable, recurrent, disturbing thoughts) and compulsions (repetitive or ritualized behaviors). Some research shows that it may have to do with serotonin, which helps to keep people from repeating the same behaviors over and over again. A person who has OCD may not have enough serotonin. Therefore, many people who have OCD can function better when they take medicines that increase the amount of serotonin in their brains.

Serotonin syndrome

Extremely high levels of serotonin can have toxic and potentially fatal effects, causing a condition known as serotonin syndrome. In practice, such toxic levels are essentially impossible to reach through an overdose of a single anti-depressant drug, but require a combination of serotonergic agents, such as an SSRI with an MAOI. The intensity of the symptoms of serotonin syndrome vary over a wide spectrum, and the milder forms are seen even at non-toxic levels. For example, recreational doses of MDMA (ecstasy) will generally cause such symptoms but only rarely lead to true toxicity.

Chronic diseases resulting from serotonin 5-HT2B overstimulation

In blood, serotonin stored in platelets is active wherever platelets bind, as a vasoconstrictor to stop bleeding, and also as a fibrocyte mitotic, to aid healing. Because of these effects, overdoses of serotonin, or serotonin agonist drugs, may cause acute or chronic pulmonary hypertension from pulmonary vasoconstriction, or else syndromes of retroperitoneal fibrosis or cardiac valve fibrosis (endocardial fibrosis) from overstimulation of serotonic growth receptors on fibrocytes.

Serotonin itself may cause a syndrome of cardiac fibrosis when it is eaten in large quantities in the diet (the Matoki banana of East Africa) or when it is over-secreted by certain mid-gut carcinoid tumors. The valvular fibrosis in such cases is typically on the right side of the heart, since excess serotonin in the serum outside platelets is metabolized in the lungs, and does not reach the left circulation.

Serotonergic agonist drugs in overdose in experimental animals not only cause acute (and sometimes fatal) pulmonary hypertension, but there is epidemiologic evidence that chronic use of certain of these drugs produce a chronic pulmonary hypertensive syndrome in humans. Some serotonergic agonist drugs also cause fibrosis anywhere in the body, particularly the syndrome of retroperitoneal fibrosis, as well as cardiac valve fibrosis.

In the past, three groups of serotonergic drugs have been epidemiolgically linked with these syndromes. They are the serotonergic vasoconstrictive anti-migraine drugs (ergotamine and methysergide), the serotonergic appetite suppressant drugs (fenfluramine, chlorphentermine, and aminorex), and certain anti-parkinsonian dopaminergic agonists, which also stimulate serotonergic 5-HT2B receptors. These include pergolide and cabergoline, but not the more dopamine-specific lisuride. As with fenfluramine, some of these drugs have been withdrawn from the market after groups taking them showed a statistical increase of one or more of the side effects described. An example is pergolide. The drug was in decreasing use since reported in 2003 to be associated with cardiac fibrosis. Two independent studies published in the New England Journal of Medicine in January 2007, implicated pergolide along with cabergoline in causing valvular heart disease. As a result of this, the FDA removed pergolide from the U.S. market in March, 2007. (Since cabergoline is not approved in the U.S. for Parkinson's Disease, but for hyperprolactinemia, the drug remains on the market. Treatment for hyperprolactinemia requires lower doses than that for Parkinson's Disease, diminishing the risk of valvular heart disease).

Because neither the amino acid L-tryptophan nor the SSRI-class antidepressants raise blood serotonin levels , they are not under suspicion to cause the syndromes described. However, since 5-hydroxytryptophan (5-HTP) does raise blood serotonin levels, it is under some of the same scrutiny as actively serotonergic drugs.

In unicellular organisms

Serotonin is used by a variety of single-cell organisms for various purposes. Selective serotonin re-uptake inhibitors (SSRIs) have been found to be toxic to algae. The gastrointestinal parasite Entamoeba histolytica secretes serotonin, causing a sustained secretory diarrhea in some patients. Patients infected with Entamoeba histolytica have been found to have highly elevated serum serotonin levels which returned to normal following resolution of the infection. Entamoeba histolytica also responds to the presence of serotonin by becoming more virulent.

History

Isolated and named in 1948 by Maurice M. Rapport, Arda Green, and Irvine Page of the Cleveland Clinic,
the name serotonin is something of a misnomer and reflects the circumstances of the compound's discovery. It was initially identified as a vasoconstrictor substance in blood serum – hence serotonin, a serum agent affecting vascular tone. This agent was later chemically identified as 5-hydroxytryptamine (5-HT) by Rapport, and, as the broad range of physiological roles were elucidated, 5-HT became the preferred name in the pharmacological field.

Increasing serotonin levels

Serotonin levels may be increased by supplement of tryptophan. However, increasing foods rich in tryptophan (eg, meats, proteins) do not increase serotonin levels, due to competition with other amino acids.
Much research has indicated that vigorous aerobic exercise improves mood, believed to be facilitated by an increase in serotonin levels. Research also suggests that eating a diet rich in whole grain carbohydrates and low in protein will increase serotonin by secreting insulin, which helps in amino acid competition. However, increasing insulin for a long period of time can sometimes onset insulin resistance, which is related to obesity, type 2 diabetes, and lower serotonin levels. It is also believed that muscles use many of the amino acids except tryptophan, allowing men to have more serotonin than women.
Bright light therapy is another popular method which prevents the conversion of serotonin to melatonin.
A similar effect is obtained by spending more time in natural sunlight.